Read a few threads but didn't feel like there was a definitive method to undervolt these cards. I have 2 6970's and two 6990's in a machine right now and would like to increase their efficiency as much as possible.

Looking for lowest stable voltage no matter what the clocks need to be and of course lowest possible memclock.

I have a box with 2x6990 and 1x6970 on stock BIOS, running at:500MHz core / 450MHz memory @ 1.0V for the 6990s500/400 @1.0V on the 6970

1120MH/s - 480W at the wall. Oh, and having fans running below 40% is nice!

There is another small voltage step to 0.95V if you drop all the way down to 275/150, but I saw not reason to run that low.

At such a low clocks you should be able to reduce the voltage much, much more.Not sure which software will work though without BIOS flash but you can give a try to all the tools available.For me barelyclocked works well for undervolting/overvolting all cards.

HD 6770 - works 24/7 stable @ 775/180 with 0.82V (stock was 775/1000 and 1.125V). This way temperatures decreased by 27C (the card is in a very hot place).

I tried underclocking my 6950 with MSI Afterburner, but it seems the memory will not underclock at all. At 1v and 700/1250 mhz the card is mostly stable but sometimes rarely it hangs up. I'm running it at those speeds with 1.025v, seems stable so far. Load temps dropped a good 10c (using a scythe cooler + noctua fan).

At 0.975v the card ran fine for about a day before locking up. At 0.950v it locked up outright when I launched a game.

I'm aiming to reduce my power bill as much as possible, without having to part with this awesome card and without giving up too much of its performance.

I've been fighting with my 3 6870s over these same issues. At this point I've tried ATI tray tools, Trixx, CGminer, and CCC. None of them will adjust the voltage and I'm stuck with the memory 100 mhz below the clock speed. I was having heat issues until I pulled my wireless card and added an extra 80mm that I had laying around. I'd like to be able to drop the memory and voltage though to increase efficiency. I'm going to try afterburner next and see if that will unlock them.